You definetly need to upgrade your intake too, that whimpy little excellerator is way to small for any serious power on a goodbreathing 406.
A victor jr would be more suited to your needs, or a newer team G.
The above mentioned sportsman II heads are really good pieces out of the box and are very affordable verses pricey aluminum heads and they would be complimented by the intakes I reccomended. Now as far a 400 not liking to rev past 6000????? It all depends on your bottom end being built right and the cam, intake and heads.
I've seen many SBC'S bigger than 406's wound up in the 7000 to 8000rpm range.
Goodluck, Charles